Novel light-responsive polymer material walks at caterpillar speed

14 July 2017

A time-lapse image of the new walking device. Image: Bart van Overbeeke.

Scientists at Eindhoven University of Technology in the Netherlands and Kent State University in the US have developed a new polymer material that can undulate and thus propel itself forward under the influence of light.

When clamped in a rectangular frame and illuminated, a strip of this polymer material can go for a walk all on its own. This small device, the size of a paperclip, is the world's first machine for converting light directly into walking, simply using one fixed light source. The researchers report this development in a paper in Nature.

The maximum walking speed of the device is equivalent to that of a caterpillar, about half a centimeter per second. The researchers think it could be used to transport small items to hard-to-reach places or to keep the surface of solar cells clean. When grains of sand were placed on the strip, they were removed by the undulating movement. The mechanism is so powerful that the strip can even transport uphill an object that is much bigger and heavier than the device itself.

The motion of the new material is due to the fact that one side contracts in reaction to light while the other one expands, causing the strip to bulge when illuminated. This deformation disappears instantaneously once the light is gone. Although the material looks transparent to the human eye, it fully absorbs the violet light the researchers used, thus creating a shadow behind it.

The scientific team, led by Dick Broer at Eindhoven University of Technology, was able to create a continual undulating movement, using this 'self-shadowing' effect. They attached a strip of the material in a frame shorter than the strip itself, causing it to bulge, and then shone a concentrated LED light on it.

The illuminated section of the strip starts to bend downward, creating a 'dent' that brings the next section of the strip into the light, causing it to deform. In this way, the 'dent' moves backwards, creating a continual undulating movement. This sets the device in motion, walking away from the light. When the device is placed upside down, the wave travels in the opposite direction, causing it to walk towards the light.

The research team produced this behavior in the material with 'liquid crystals', by incorporating a derivative of a fast-responding, light-sensitive molecule known as azobenzene in a liquid crystalline polymer network. They engineered the material so that azobenzene’s light-sensitive response is translated into an instantaneous deformation of the strip when illuminated, followed by relaxation as soon as the light is removed.

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